Extensional wave attenuation and velocity measurements on a high permeability Monterey sand were performed over a range of gas saturations for imbibition and degassing conditions. These measurements were conducted using extensional wave pulse propagation and resonance over a 1 9 kHz frequency range for a hydrostatic confining pressure of 8.3 MPa. Analysis of the extensional wave data and the corresponding Xray CT images of the gas saturation show strong attenuation resulting from the presence of the gas (QE dropped from 300 for the dry sand to 30 for the partially-saturated sand), with larger attenuation at a given saturation resulting from heterogeneous gas distributions. The extensional wave velocities are in agreement with Gassmann theory for the test with near-homogeneous gas saturation and with a patcry saturation model for the test with heterogeneous gas saturation. These results show that partially-saturated sands under moderate confining pressure can produce strong intrinsic attenuation for extensional waves.
The acoustic properties of poorly-consolidated sands is a topic of importance in a number of fields, including geotechnical investigation of soil properties for stability (Stoll, 1989; Ishihara, 1996), environmental monitoring of contaminants in the shallow subsurface (Geller and Myer, 1995; Seifert et al., 1998), and geophysical characterization of sand reservoirs and aquifers for petroleum production (Gardner et al., 1964) and CO2 sequestration (Eiken et al., 2000).
Poorly-consolidated sands can be viewed as an end-member of the spectrum of naturally-occurring granular materials, with tight sandstones as the other end-member. From a macroscopic scale, poorly consolidated sand might appear as a deceptively simple porous granular material. However, the strong stress sensitivity of sand packings, along with the added complexities resulting from the presence of clay, and multiple fluid and gas phases, conspire to make the present understanding of this class of materials incomplete.
Systematic studies of these effects over a broad frequency-range are required for a comprehensive understanding of the acoustic properties of poorly consolidated sands. The overall objective of our research program on sands is to investigate the role of partial gas saturation on the attenuation and velocity of acoustic waves in the seismic to sonic frequency range (1 Hz to 20 kHz) over a range of confining pressures. As a first step towards this goal, we have developed a sonic frequency apparatus that utilizes resonance and pulse propagation to measure the velocities and attenuation of poorly consolidated sands under hydrostatic confinement. The confining vessel is fabricated of aluminum, allowing the gas and fluid phases to be imaged with an X-ray CT scanner. In this paper, we report our recent efforts to measure the extensional wave velocities and attenuation of a Monterey sand with homogeneous and heterogeneous distributions of gas.
